In order to produce ethylene and other lower olefins, hydrocarbons or mixtures of hydrocarbons are thermally cracked, for example in externally heated reactors formed of metallic materials and the hot cracked products obtained thereby are cooled after leaving the cracking furnace in heat exchanger apparatuses which are operated externally with water under pressure serving as coolant.
The cracking furnaces are preferably formed of high-temperature steels containing chromium and nickel. The tubular heat exchangers are preferably formed of low-alloy steels or boiler construction steel. This apparatus can also be used to produce other organic products, e.g., as in the production of vinyl chloride by pyrolysis of 1,2-dichloroethane.
The operating efficiency of such apparatus formed of metallic materials is highly dependent on the extent of carbon-rich deposits forming at their inner surfaces during operation. Such deposits can not only impede the desired heat transfer, but can also reduce the free cross section of the employed tubes which is important for maintaining throughput. This is true of currently used apparatus 25. FIG. 1 shows a typical curve A for the dependence of the quantity of deposited coke-like products m on the reaction time t.
After a certain period of operation, the deposits formed on the sides of the apparatus coming into contact with the organic compounds reach a permissible coke layer thickness S, as shown in FIG. 1, which causes reductions in output and necessitates the shutdown of operations and costly cleaning procedures. The coke-like deposits are usually removed by gasification using a mixture of hot steam and air which uncovers the metallic surfaces and ensures the desired heat flow.
In spite of thorough removal of the deposited coke, the newly forming deposits can again lead to compulsory shutdown and coke removal procedures already after a relatively short period of operation (e.g., 20 to 60 days). Since the applied oxidative decoking procedures simultaneously bring about a change in the material surfaces, such decoking procedures always involve an increase in the catalytic activity of the material surfaces which promotes unwanted surface coking. This catalytic activity increases with the number of decoking procedures to which the respective heat exchange surface is subjected and the operating periods between decoking procedures decline steadily. This is undesirable for technical reasons as well as from an economic viewpoint because it not only prevents maximum periods of stationary operating states, but also reduces the effective use of the installation and results in increasingly frequent cleaning costs. For these reasons, efforts have been made for years to find solutions for preventing rapid coking of the inner surfaces of such apparatus. In order to achieve this objective, it has been suggested, among other things, to prevent the formation of catalytically active centers or to inhibit such formation on the inner surfaces of tubes of the respective apparatus by developing passivating oxide coats, as described in U.S. Pat. No. 3,919,073; to coat the inner walls of the tubes with thin coats of low-alloy or nickel-free steels, as described in German patent publication DE-A 3 2476 568, to generate supporting layers or diffusion layers of chromium, as described in the publication by Brown, S. M. and Albright, L. F. ACS Symp. Ser. 32 (1976) 296, aluminum, as described in the publication by Frech, K. J., Hopstock, F. H. and Hutchings, D. A. ACS Symp. Ser. 32 (1976) 197, or silicon, as described in the publications by Brown, D. E., Clark, J. T. K., Foster, A. J., McCaroll, J. J. and Simms, M. L. ACS Symp. Ser. New York 202 (1982) 23; Bach, G., Zychlinski, W., Zimmermann, G., Kopinke, F. D. and Anders, K. Chem. Techn. Leipzig 42 (1990) 146; Ansari, A. A., Saunders, S. R. J., Bennett, M. J., Tuson, A. T., Ayers, C. F. and Steen, W. M. Materials Science and Engineering 88 (1987) 135; and to add additives in the form of gas or steam of sulfur-containing compounds, as described in the publication by Boene, K. Oilgas J. 81 (1983) 93, phosphorus-containing compounds, as described in the publication by Gosh, K. K. and Kunzru, D. Ind. Engng. Chem. Res. 27 (1988) 559 and in U.S. Pat. Nos. 4,835,332; 4,842,716; and 4,900,426, and nitrogen-containing compounds, as described in the publication by Egiasarov, J. G., Cores, B. Ch. and Potapova, L. L. "Neftechimija." Erdolchem 25 (1985) 627, to the charging product.
As disclosed in U.S. Pat. Nos. 4,835,332; 4,842,716; and 4,900,426 it is known to reduce the formation of coke-like deposits on the inner surfaces of reactors by adding organic phosphorus compounds. The organic phosphorus compounds (including organic thiophosphorus) can be used as such or as constituents of special compounds. The addition of organic phosphorus compounds is always linked with the formation of more or less volatile phosphines which are not only toxic but can also lead to catalyst contamination in the downstream processes. The addition of organic phosphorus compounds is effective only within a limited scope.
Contradictory assertions have been made, such as those disclosed in Czechoslovakian patent publication CS-A 180861 and in the publication by Froment, G. F. Reviews in Chem. Eng. 6(4) (1990) 293, concerning the effect of sulfur compounds on coking. Nevertheless, sulfur compounds are frequently used in industrial practice hydrocarbon fractions (naphtha, kerosine, gas oil, etc.), the addition of sulfur compounds has hardly any discernable effect on coking. They contain ad hoc sulfur compounds as mixture components. However, a more or less pronounced formation of coke-like deposits is observed during the pyrolysis of such hydrocarbon fractions.
In addition, although the application of oxidic protective coatings, as is suggested, in European patent publication EP-A 0 110 486, would lead to improvements, it cannot be considered a satisfactory solution.
A further improvement is provided by a coating based on silicon oil which is subsequently thermally decomposed under strictly specified conditions to produce a protective layer, as described in the publication Chem. Tech. Leipzig 42 (1990) 146. This process, like the production of laser-induced SiO.sub.2 surface layers, is relatively costly and the generated SiO.sub.2 layers are not stable during changes in the temperature of the outer tube wall in the range of 750 to 1100.degree. C. This also applies to any passivated layers obtained by the silica coating which is described by British Petroleum Co. Ltd. in the publication ACS Symp. Ser. New York, 202 (1982) 23-43 in comparison with the publication Chem Techn. Leipzig 42 (1990) 146 ff.
Finally, reference is made to the attempted use of tubes of steel alloys whose inner surface is coated by thin coats of low-alloy or nickel-free steels described in German patent publication DE-A 3 247 568. It has been shown that the results of such plating do not justify the effort.
With the exception of the reduction of coke formation through the addition of phosphorus- and/or sulfur-containing additives to the pyrolysis charging products, all of the proposed solutions described above can only be practically carried out in new installations or in new tubing, but not in installations which have already been in use.
Therefore, the object of the present invention is to propose new improved heat exchange surfaces and to provide a process for reducing coking by which the respective apparatus (outfitting) of an installation which has already been completely installed can be subjected to such treatment before being put into operation and also after every decoking procedure.